Recording materials using an electron-donating colorless dye (hereinafter referred to as a "color former") and an electron-accepting compound (hereinafter referred to as a "developer") are already well known as a pressure-sensitive paper, a heat-sensitive paper, a light-sensitive and pressure-sensitive paper, an electric heat-sensitive paper and the like. Such papers are disclosed in detail, for example, in British Pat. No. 2,140,449, U.S. Pat. Nos. 4,480,052, 4,436,920, 3,775,424, 4,181,328 (British Pat. No. 1,552,517), European Pat. No. 82822, Japanese Patent Application (OPI) Nos. 179836/82, 123556/85 and 123557/85 (the term "OPI" as used herein means an "unexamined published Japanese patent application").
In a good recording material, (1) both color density and color sensitivity should be sufficient, (2) fog should not occur, (3) color images should have sufficient color fastness, (4) the hue of color images should be appropriate and suitable for duplicating, (5) the S/N ratio should be high, (6) the chemical resistance of color images should be sufficient, and (7) the recording material should easily be dissolved by an organic solvent. So far, satisfactory recording materials meeting all of the above requirements completely have not been realized.
Recently, with the varied demands for recording systems, extensive research to improve these characteristics has ben conducted. Particularly, sufficient color fastness is in great demand for blue color recording materials.
Diphenylmethane type compounds, triphenylmethane type compounds, phthalide type compounds, leucomethylene blue type compounds and the like are conventionally known as compounds which form a color from blue to bluish-purple. However, each of these compounds has disadvantages. For example, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (i.e., crystal violet lactone) quickly forms a dark blue color, but the light fastness of color images is extremely poor. 3,7-bis(dimethylamino)-10-benzoylphenothiazine (i.e., benzoylleucomethylene blue) as a leucomethylene blue type compound provides excellent light fastness in color images, but color formation is very slow, and in combination with an organic developer color formation is extremely inferior. Furthermore, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4- or 7-azaphthalide which is known as a phenylindolylazaphthalide type compound has poor solubility in a solvent used for encapsulation, and self-color-developing properties are very strong.